1. Field of the Invention
This invention relates to FM signal converters and optical transmitting apparatus for use in telecommunications, CATVs, measurements, mobile communications and the like.
2. Releted art of the Invention
Recently, multi-channel optical transmission of video picture, voice or data that exploits low-loss broadband characteristics of optical fibers is made available in video monitoring systems, CATVs, subscriber lines and mobile communications. In such optical transmissions, multi-channel signal is electrically multiplexed by a plurality of sub-carriers with different frequencies into AM-SCM signal. The multiplexed signal is converted to optical signal by directly modulating e.g., semiconductor lasers to transmit via optical fiber.
Video optical transmission of AM signal is not costly because the modem configuration is simple. However, in order to achieve a C/N (carrier to noise ratio) associated with a desired video quality, high optical input power is required in the receiver side. Also, in mobile communication, since intensity level of transmitted voice and data signal vary substantially when a mobile terminal moves, high dynamic range against the signal variation is required. Further, distortion caused by reflected wave during optical conversion of semiconductor lasers and during optical fiber transmission may effect much, and an amplifier of AM signal requires good linear characteristics.
Following above needs, in order to improve anti-distortion and anti-noise of the optical transmission, an approach for optical transmission that converts subcarrier multiplexed AM signal together into FM signal is proposed. In order to get desired C/N value in optical transmission, modulation index is required to be large.
FIG. 9 shows a configuration of a prior art FM signal optical transmitting apparatus. FIG. 9 shows an optical transmitter 111, an FM modulator 101, a driver amplifier 201, a semiconductor laser 210, optical fiber 27, an optical receiving unit 22, a photodetector 23, a preamplifier 24, an FM demodulator 25 and a low-pass filter 26.
Operation of such a prior art FM signal optical transmitting apparatus with such a configuration is as follows. Multi-channel frequency multiplexed AM video signal (AM-SCM signal) is provided to an FM modulator 101 and broadband FM modulated signal is outputted.
As shown in FIG. 7(a), when the FM modulator directly modulates an FM laser (FM-LD) 81 via AM signal, not only amplitude of light is modulated but oscillating frequency of the light is also modulated. After this light is multiplexed by a multiplexer 83 with a local light source (LO-LD) 82 that has slightly different oscillating frequency, the light is provided to a photodiode 84 to be optical heterodyne detected, and broadband (for example, 1 to 6 GHz) FM modulated signal is obtained as the beat signal of two lasers. Using this FM modulated signal, a transmitting semiconductor laser 210 is directly modulated via a laser driving amplifier 201, and the signal is converted to optical signal to be transmitted via optical fibers 72. (For example, refer to Japanese Patent No. 2700622.)
Then, after the optical signal is amplified by e.g., optical amplifier, the signal is transmitted to a respective receiving unit via an optical branching unit. Illustratively, only one receiving unit 22 is shown. In the receiving unit 22, after the optical signal is converted to electrical signal by the photodetector 23 and is amplified by the preamplifier 24, the signal is demodulated into original AM-SCM signal in the FM demodulator 25 and a low pass filter. For example, the FM demodulator 25 is a delayed demodulating circuit and is composed of a high-speed logic IC (ex. AND gate) and a delaying unit, and a limiter amplifier.
As described, in prior art FM signal optical transmitting apparatus, in order to achieve the desired carrier-to-noise ratio (CNR) by FM modulation and demodulation, multi-channel AM-SCM signal with large amplitude variation is provided to an FM modulator since large frequency deviation is required. Multi-channel signal like this multi-channel AM-SCM signal generates large amplitude variation instantaneously if there are no correlation between each of the signals.
Therefore, in modulating operation of the FM modulator 101, an instantaneous amplitude variation of a voltage or a current of the AM-SCM signal directly becomes same with the frequency deviation.
However, in FM modulators in general, oscillating frequency deviation does not have linear characteristic against input amplitudes. As shown in FIG. 8, regarding curve 71 that denotes oscillating frequency deviation against amplitude variation, suppose the instantaneous amplitude variation 61 had a large peek value at time xe2x80x9ctpxe2x80x9d instantaneously, it may exceed a range (xe2x80x9cA0+Athxe2x80x9d in FIG. 8) that the relationship between the amplitude variation and the oscillating frequency signal deviation remains linear, and may be FM modulated in non-linear area.
If these FM modulated signal is demodulated, modulated components in the above non-linear area may appear as a distortion in the demodulated signal, and aggravate the co-modulation distortion of the AM-SCM signal.
An object of the present invention is to provide a low cost FM signal optical transmitter that has a simple configuration, a low distortion and a good receiver sensitivity and that can transmit multi-channel AM-SCM signal, and an FM signal converter for enabling the above mentioned FM signal optical transmitter, and a low cost FM signal optical receiver.
An FM signal converter of the present invention comprises:
an amplitude detecting unit for detecting amplitude variation of a plurality of signals that are multiplexed with subcarriers, as an amplitude variation signal;
a peak detection unit for determining from said amplitude variation signal whether a peak of the amplitude of said plurality of signals exceeds a threshold and for generating peak detection information that includes information about said peak of the amplitude;
a frequency signal source for providing signal with a predetermined frequency that differs from any of the frequencies of said subcarriers;
an amplitude phase control unit for adjusting amplitude and phase of the signal from the frequency signal source according to said peak detecting information and outputting the adjusted signal as a corrected signal;
signal combining means for combining said corrected signal and said plurality of signals multiplexed with subcarriers, with considering a time for generating the corrected signal; and
an FM modulator form modulating said signal from the signal combining means into an FM signal.
The FM signal converter according to said present invention is also such that
the plurality of signals multiplexed with subcarriers can be obtained by suppressing a component of a carrier having said predetermined frequency out of a signal demodulated from said FM signal.
The FM signal converter according to said present invention is also such that
said amplitude phase control unit adjusts amplitude and phase of the signal from the frequency signal source in such manner that amplitude level of voltage or current of said corrected signal is smaller than said predetermined threshold.
The FM signal converter according to said present invention is also such that
said predetermined threshold is determined according to oscillating frequency deviation characteristics of said FM modulator.
The FM signal converter according to said present invention is also such that
the signal outputted by said frequency signal source has at least one carrier frequency.
The FM signal converter-according to present invention is also such that
the signals outputted by said frequency signal source are such signals that is converted from said plurality of signals multiplexed by subcarriers so that frequency band of converted signals do not overlap with those of said plurality of signals.
The FM signal converter according to said present invention is also such that
said signals having at least one carrier frequency are a plurality of signals-with different phases.
The FM signal converter according to said present invention is also such that
the signals outputted by the frequency signal source are signals converted from said plurality of signals multiplexed by subcarriers so that phase of converted signals do not overlap with those of said plurality of signals.
The FM signal converter according to said present invention can further comprise:
a signal switching unit for selectively outputting the signal outputted by the frequency signal source according to the magnitude of said peak of the amplitude.
The FM signal converter according to said present invention is also such that
said signal switching unit selectively outputs a signal with optimum phase out of signals provided from said frequency signal source.
The FM signal converter according to any one of said present inventions is also such that
a voltage-controlled oscillator is provided as said FM modulator.
The FM signal converter according to any one of said present inventions is also such that
two semiconductor laser devices are provided as said FM modulator each which oscillates with single mode frequency and frequencies which are oscillated from said tow laser devices are proximate to each other, and further comprises:
an optical detector for O(optical)/E(electric) converting such multiplexed signal that is obtained,
by directly modulating output signal from said signal combining means by one of said semiconductor laser device and multiplexing the modulated signal with signal from the other said semiconductor laser device, or
by dividing output said signal from said signal combining means into two signals, and multiplexing such signal which is generated by directly modulating one of said divided signals by one of said semiconductor laser device, with such signal which is generated by phase-inverting the other dicided signal and further directly modulating the phase-inverted signal by the other semiconductor laser device.
An FM signal optical transmitter of the present invention comprises:
the FM signal converter according to any one of said present inventions; and
an E/O converting unit for converting signal outputted from the FM signal converter into optical signal.
An FM signal optical receiver of the present invention comprises:
an O/E converting unit for converting optical signal transmitted from the FM signal optical transmitter according to claim 13 into electrical signal;
an FM demodulator for demodulating the FM signal converted into electrical signals; and
a filter for outputting a plurality of signals multiplesed with subcarriers by suppressing a component of a carrier having said predetermined frequency out of the signal from the FM demodulator.